This invention relates to improvements in the rate of melting glass in a tank-type melting furnace whereby the output of a particular furnace may be increased at a given energy consumption, or, conversely, the energy consumption may be reduced for a given throughput. More particularly, the invention deals with improvements in the manner in which raw glass batch materials are fed to a glass melting furnace so as to enhance the rate the raw ingredients are brought to a liquid state.
In a typical glass melting furnace of the regenerative or recuperative type, a body of molten glass is maintained in the furnace and raw glass batch materials are fed through an inlet at one end of the furnace onto the surface of the pool of molten glass. There, the batch materials usually form an unmelted layer on the surface of the molten glass pool which may extend a considerable distance into the furnace until it becomes melted into the pool of molten glass. At the opposite end of the furnace, melted and reacted glass is withdrawn from the pool of molten glass through an outlet opening.
It has been recognized that the floating layer or blanket of unmelted batch ingredients acts as a thermal insulator which limits the rate at which the temperature of the batch is raised sufficiently to enter a liquid state. Therefore, liquefaction of glass batch usually is limited to a relatively thin layer at the surface of the batch blanket. In order to overcome this problem, attempts have been made in the past to increase the surface area of the batch blanket exposed to the flames in the furnace. For example, U.S. Pat. No. 4,030,905 shows an arrangement for plowing furrows transversely across a batch blanket. Such an arrangement may produce an increase in batch surface area and some slight improvement in run-off of melted batch, but possesses certain drawbacks. Plowing the furrows causes batch to be piled up more deeply on either side of each furrow, thereby further insulating the underlying batch from the overhead sources of heat. Furthermore, any enhancement in run-off by plowing is limited because the furrows do not extend to the underlying molten glass and because some of the loose batch material tends to fall back into the furrow behind the plow.
Another approach to breaking up a batch blanket is disclosed in U.S. Pat. No. 3,994,710 wherein an inverted T shaped member is employed to chop the batch blanket into pieces. Such an arrangement appears most suitable for a location relatively far into the furnace where melting of the batch blanket has already progressed to an advanced stage. It would be desirable to improve run-off as early as possible in the melting process. Additionally, by being located within the main body of the melting furance, the T bar of the patent requires cooling which detracts from any net thermal gains. Also, operating on the batch blanket within the main body of the furnace carries with it the risk of increased carry-over of materials which can have an adverse effect on the walls and regenerator or recuperator system of the furnace. However, carrying out such a chopping operation on an upstream portion of the batch blanket would not appear to be advantageous since the buoyant batch material would be pressed into the molten glass temporarily and then rise again.
Another prior art approach has been to bring the batch ingredients into more intimate contact with the molten glass such as in U.S. Pat. Nos. 2,533,826 and 2,749,666. The object of this approach is to take advantage of conductive heat from the molten glass, but it has now been found that the major source of heat (typically about seventy percent) for melting the batch is the overhead radiant heat from the combustion flames in the furnace. Therefore, covering the batch with molten glass can be disadvantageous in that it reduces the amount of radiant heat received by the batch. It would be desirable to increase rather than decrease the impingement of radiant energy on the batch materials.
Other attempts have been made to improve batch melting by reducing the thickness of the batch blanket such as in U.S. Pat. Nos. 2,327,887; 3,193,119; and 4,004,903. While reducing batch blanket thickness may generally be desirable, the approach in each of these patents has the drawback of reducing surface area exposed to overhead flames and inhibiting run-off of melted batch. Furthermore, in many commercial glass melting operations, a primary objective is to maximize throughput of a given furnace. In such a case, the batch blanket would already cover a maximum area and any reduction in batch blanket thickness would undesirably reduce the throughput of the furnace. The last mentioned patent overcomes this dilemma somewhat by compacting the batch blanket, but, nevertheless, a flat upper surface is the result.
It is also known to produce a plurality of discrete batch piles by employing a plurality of small batch feeders such as in U.S. Pat. No. 3,127,033. Such an approach appears to be quite limited as to throughput because of the small size of the inlets through which batch is fed.
Two types of batch feeders are in widespread commercial use in the glass industry. The first being the reciprocating tray type as shown in U.S. Pat. Nos. 1,916,262 and 3,780,889 and the second being the rotary type as shown in U.S. Pat. No. 2,829,784. The reciprocating tray type feeder inherently tends to form a series of ridges extending laterally across the batch blanket. However, these ridges are not as steep as would be desired for the sake of enhancing run-off nor do the furrows between the ridges provide a sufficiently free path for run-off. After melting of the batch blanket has progressed substantially, the ridges typically become separated into floating masses known as "logs." However, break-up of the batch blanket does not occur as early as would be desired. The rotary type feeder produces a nearly level batch blanket with only a shallow treadmark on the surface produced by the rotary feeder blades. Hence, the rotary type feeder is particularly characterized by poor run-off.
While the prior art appears to recognize some advantages for increasing the surface area of the batch blanket and for minimizing the thickness of the batch layer, these improvements have heretofore been implemented in embodiments which favor one of the improvements to the exclusion of the other. Furthermore, it appears that the prior art has not fully appreciated nor used the advantages attendant to enhancing runoff of melted material from a batch blanket.